A Ring back tone (RBT) is an audible indication that is heard by the caller in a telephonic communication while the caller's phone is still ringing. It is normally a repeated tone, designed to assure the caller that the called party's line is ringing.
The RBT may be generated in the distant switch and transmitted in-band. In analog networks the caller could therefore monitor the quality of the complete voice path of the connection before the call was established. The distant switch also sends a message out-of-band indicating to the rest of the following network that the phone is ringing. Under strict implementation of certain protocols and signaling systems, the closest switch to the caller generates the RBT. In most public phone networks the RBT is not generated in the handset or by the local switch, as customized tones or voice announcements may be generated by the distant switch in place of a ringing signal. Conventionally, the operator generated RBTs plays on for nearly 8 s-10 s and the convention ‘tring-tring’ or other caller tones leaves little room for customization and user enhancement.
In RBT application in its current form, there is a fixed catalogue of short RBT clips (30 s duration), provided by music labels that the end-user chooses to play as RBT. However, this approach restricts user choices of music because the most meaningful part of the song can be anywhere in the song.